There’s an old joke that all guys wear large-sized condoms. Not sure? Ask any guy you know if he wears a small, medium, or large, and he’ll tell you he wears an extra-large.

Author Michael Gurian says most boys have questions about penis size during adolescence but rarely ask them. Unlike girls’ concerns about breast size, there’s no easy way for a guy to know where his erection ranks in comparison to his peers.

Porn, gay and straight, gives guys a chance to see someone else’s erect penis. Of course, those guys are usually hung like horses, so that’s not particularly helpful.

Through experience, guys who have sex with other guys eventually get a pretty good sense of how they measure up. But guys who only have female partners don’t get that. Can you imagine a group of straight guys in a room together, playing with themselves long enough to get hard but not climaxing, then standing next to each other to see who’s bigger?

A Starbucks Grande coffee cup is not quite six inches tall. So is the average erect penis.

The easy way to determine if you’re close to average is to go to Starbucks. Seriously. A Grande coffee cup is not quite six inches tall. So is the average erect penis; you can see the stats here.

Most women will tell you that length doesn’t matter very much for penis-in-vagina sex. It only matters when a penis is particularly short or especially long.

The real key is girth. The thicker the penis, the better it will be at creating friction.

Thickness is also the key dimension in choosing the correct condom. Unless you’re especially long, any condom should give you plenty of coverage. If there’s extra condom rolled up at the base of your shaft, that’s not a problem.

Wayne Pawlowski is a certified sexuality educator (via AASECT, the American Association for Sexuality Educators, Counselors, and Therapists) and Licensed Clinical Social Worker. He recently presented at the Center for Family Life Education’s National Sex Ed Conference, where he shared a new piece of lore. He’s also seen this in professional publications and has recommended it to many of the young men he’s worked with. At the conference, he shared it with a room full of about 60 sexuality educators, none of whom had ever heard it before.

In order to determine what size condom you need, use the cardboard tube from a roll of toilet paper.

Yes, really. Regular-sized condoms have almost the same circumference as that toilet paper tube. Condoms are a shade smaller, but a rolled up condom is as big around as the end of that toilet paper tube.

If the thickest part of your penis—the base of the head—can fit into that tube without much effort, then a regular condom will fit you well. If the thickest part of your penis has plenty of room inside that tube, then you need small-sized condoms, sometimes called “snugs.” And if your head doesn’t fit through that toilet paper roll without a whole lot of effort or pain, then you need the large or “magnum” size.

Having a properly fitted condom is important. If your condom is too small, you probably won’t be able to put it on. If it’s too big, it’ll be too loose, and then:

It won’t keep your gizz inside, which means you’ll be at risk for having a pregnancy you were trying to avoid.

It won’t keep your gizz inside, which means there’s a greater chance of giving your partner a sexually transmitted infection (STI) if you have one.

It won’t stay in place as it should, which means you’re at greater risk of skin-to-skin contact and thus at greater risk for both spreading and catching an STI, like HIV/AIDS or Herpes.

If you’re wondering what else you don’t know about condoms, check out Planned Parenthood’s condom page or this downloadable pamphlet from the Society for the Scientific Study of Sexuality.

Protect your self and protect your partner. Protect your and your partner’s health and futures. Wear the right sized condom. There’s no good reason not to.